Carbon dioxide partial pressures exceeding about 7.6 mm Hg (millimeters of mercury; partial pressure of about 1%), for extended periods of time, are known to cause health problems in humans and other mammals. As a result, carbon dioxide partial pressures are typically maintained below about 1% via the use of regenerable and nonregenerable carbon dioxide sorbents such as soda lime, molecular sieves, solid oxide sorbents, alkali metal carbonates, alkali metal hydroxides, amine sorbents, and combinations thereof, among others.
In the enclosed environment provided by a submarine, amines, such as monoethanolamine and diethanolamine, are often used to reduce carbon dioxide partial pressures via absorption. These amines are utilized in the aqueous phase, typically 25 wt. % (weight percent) to 30 wt. % amine. The amine solution enters the top of an absorption tower while the carbon dioxide containing gaseous stream is introduced to the bottom. While intimately contacting the gaseous stream in a countercurrent fashion, the amine solution chemically absorbs the carbon dioxide from the gaseous stream. Desorption of the absorbed carbon dioxide then proceeds via a thermal regeneration process at temperatures in excess of about 150.degree. F. (about 66.degree. C.). During desorption, carbon dioxide and water evolve from the amine solution and are separated by condensing the water vapor in a heat exchanger. Once regenerated, the amine solution is recycled back to the absorption tower for additional carbon dioxide absorption.
Although this amine solution is capable of initially removing carbon dioxide to partial pressures below about 1%, it has a limited life due to degradation through oxidation of the amine. Oxygen present in the gas stream oxidizes the amine particularly during desorption when the sorbent is heated. This oxidation is believed to reduce the amount of amine primary and secondary functional groups available for carbon dioxide absorption. Consequently, the amine solution's useful life is limited to only about six months of continuous use.
In contrast, if desorption is accomplished at ambient temperatures, the amine solution's life will be extended, however, performance will be limited by low desorption rates. Due to both energy requirements and oxidation related degradation, amine sorbents utilized in closed environment systems are often regenerated at approximately ambient temperatures for a fixed desorption time. At ambient temperatures, the desorption of carbon dioxide is often limiting. Under these conditions, the desorption process will typically have insufficient time to fully desorb the absorbed carbon dioxide. Consequently, a portion of the absorbed carbon dioxide remains in the sorbent after the desorption process is complete, thereby reducing the capacity of the sorbent to absorb additional carbon dioxide. As a result, a decreasing portion of the carbon dioxide sorbent is used throughout the absorption-desorption cyclical process.
What is needed in the art is a regenerable carbon dioxide sorbent having high carbon dioxide removal rates and high desorption rates at ambient temperatures, and having a sorbent life exceeding about 2 years of continuous use.